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| {{Redirect|Ocean swell|the thoroughbred racehorse|Ocean Swell}}
| | Nice to meet you, I am Marvella Shryock. Years in the past he moved to North Dakota and his family members loves it. To gather badges is what her family members and her enjoy. Bookkeeping is my occupation.<br><br>Take a look at my web blog ... over the counter std test ([http://xrambo.com/blog/191590 Click at xrambo.com]) |
| [[Image:Early 90's Bangs.jpg|thumb|Breaking swell waves at [[Hermosa Beach]], [[California]]]]
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| [[Image:Easterly swell, Lyttelton Harbour, 29 July 2008.jpg|thumb|Swell near [[Lyttelton Harbour]], New Zealand]]
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| [[Image:Ile de ré.JPG|thumb|Swell near the Whales Lighthouse, [[Île de Ré]]]]
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| A '''swell''', in the context of an ocean, sea or lake, is a series of [[wind wave|surface gravity waves]] that is not generated by the local wind. Swell waves often have a long [[wavelength]] but this varies with the size of the water body, e.g. rarely more than 150 m in the Mediterranean, and from event to event, with swells occasionally longer than 700 m away from the most severe storms. Swells have a narrower range of [[frequency|frequencies]] and directions than the [[wind sea]], because swell waves have dispersed from their generation area and have been dissipated.
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| == Swell dissipation ==
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| The dissipation of swell energy is much stronger for short waves, which is why swells from distant storms are only long waves. The dissipation of waves with periods larger than 13 s is very weak but still significant at the scale of the Pacific Ocean.<ref>''Observation of swell dissipation across oceans'', F. Ardhuin, Collard, F., and B. Chapron, 2009: Geophys. Res. Lett. 36, L06607, {{doi|10.1029/2008GL037030}}</ref> These long swells lose half of their energy over a distance that varies from over 20000 km (half the distance round the globe) to just over 2000 km.
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| This variation was found to be a systematic function of the swell steepness: the ratio of the swell height to the wavelength. The reason for this behaviour is still unclear but it is possible that this dissipation is due to the friction at the air-sea interface.
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| == Swell dispersion and wave groups ==
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| Swells are often created by storms thousands of [[nautical mile]]s away from the beach where they break, and the propagation of the longest swells is only limited by shorelines. For example swells generated in the Indian Ocean have been recorded in California after more than half a round-the-world trip.<ref>''Directional recording of swell from distant storms'', W. H. Munk, G. R. Miller, F. E. Snodgrass, and N. F. Barber, 1963: Phil. Trans. Roy. Soc. London A 255, 505</ref> This distance allows the waves comprising the swells to be better sorted and free of [[wikt:chop|chop]] as they travel toward the coast. Waves generated by storm winds have the same speed and will group together and travel with each other, while others moving at even a fraction of a metre per second slower will lag behind, ultimately arriving many hours later due to the distance covered. The time of propagation from the source ''t'' is proportional to the distance ''X'' divided by the wave period ''T''. In deep water it is <math>t = 4 \pi X /( g T) </math> where g is the acceleration of gravity. For a storm located 10000 km away, swells with a period ''T''=15 s will arrive 10 days after the storm, followed by 14 s swells another 17 hours later, and so forth.
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| This dispersive arrivals of swells, long periods first with a reduction in the [[Frequency#Period versus frequency|peak wave period]] over time, can be used to tell the distance at which swells were generated.
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| Whereas the sea state in the storm has a [[frequency spectrum]] with more or less always the same shape (i.e. a well defined peak with dominant frequencies within plus or minus 7% of the peak), the swell spectra are more and more narrow, sometimes as 2% or less, as waves disperse further and further away. The result is that wave groups (called sets by surfers) can have a large number of waves. From about seven waves per group in the storm, this rises to 20 and more in swells from very distant storms.
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| == Swell and coastal impacts ==
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| Just like for all [[wind wave|water waves]] the energy flux is proportional to the significant wave height squared times the [[group velocity]]. In deep water this group velocity is proportional to the wave period. Hence swells, with usually longer periods, can pack a lot more energy than shorter [[wind seas]]. Also, the amplitude of [[infragravity waves]] increases dramatically with the wave period (typically like the period squared), which results in
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| higher [[run-up]].
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| As swell waves typically have long wavelengths (and thus a deeper wave base), they begin the refraction process (see [[water waves]]) at greater distances offshore (in deeper water) than locally generated waves.
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| <ref>[http://www.stormsurf.com/page2/tutorials/wavebasics.shtml Wave Basics (Stormsurf)<!-- Bot generated title -->]</ref>
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| Since swell-generated waves are mixed with normal sea waves, they can be difficult to detect with the naked eye (particularly away from the shore) if they are not significantly larger than the normal waves. From a [[signal analysis]] point of view, swells can be thought of as a fairly regular (though not continual) wave signal existing in the midst of strong noise (i.e., normal waves and chop).
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| == Swell and navigation ==
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| Swells were used by [[Polynesia]]n navigators to maintain course when no other clues were available, such as on foggy nights.<ref>[http://www.museum.upenn.edu/Navigation/putting/putting.html Micronesian Navigation - University of Pennsylvania Museum of Archaeology and Anthropology]</ref>
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| ==See also==
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| {{portal|Underwater diving}}
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| ==References==
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| {{reflist}}
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| ==External links==
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| * [http://www.coastalwatch.com.au/ Coastalwatch (Australian swell forecasts)]
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| * [http://magicseaweed.com/UK-Ireland-MSW-Surf-Charts/1/ Magicseaweed (UK swell forecasting) ]
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| * [http://www.seabreeze.com.au/ Seabreeze (Australian swell forecasts)]
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| * [http://www.swellnet.com.au/ Swellnet (Australian swell forecasts)]
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| * [http://www.stormsurf.com/page2/tutorials/wavebasics.shtml Stormsurf: Wave Basics (How swells are formed and measured)]
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| * [http://www.epa.qld.gov.au/environmental_management/coast_and_oceans/waves_and_storm_tides/wave_monitoring/waverider_buoys/ Waverider Buoys (Australian Swell Measuring Devices)]
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| {{physical oceanography}}
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| {{Diving medicine, physiology and physics}}
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| [[Category:Surfing]]
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| [[Category:Water waves]]
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Nice to meet you, I am Marvella Shryock. Years in the past he moved to North Dakota and his family members loves it. To gather badges is what her family members and her enjoy. Bookkeeping is my occupation.
Take a look at my web blog ... over the counter std test (Click at xrambo.com)